Automatic train control (ATC), which is a class of signaling safety systems for railways, requires rolling stock to receive speed signals transmitted from outside (ATC ground device installed on a ground side). Rolling stock is provided with an ATC antenna for receiving the speed signals. The ATC antenna is installed below the floor on the front portion of the lead car of rolling stock. The ATC antenna includes two antenna coils. These two antenna coils are disposed on the front side of a lead bogie and are directly above left and right rails.
For example, in an on-board device disclosed in FIG. 1 of Patent Literature 1, an ATC antenna is disposed below the floor on the front portion of the body of the lead car. Thus, the ATC antenna receives signals from the ATC ground device installed between the rails. Although Patent Literature 1 does not specifically disclose the ATC antenna as the on-board device, the ATC antenna typically includes left and right antenna coils as mentioned above.
The ATC ground device is typically a rectangular loop-shaped coil. This coil is installed such that the loop is horizontal between the left and right rails. The coil includes a coil side that is close to and parallel to the left rail and a coil side that is close to and parallel to the right rail. When an alternating current (AC) having a frequency corresponding to the speed signal is passed through the coil of the ATC ground device, AC magnetic flux in accordance with the frequency of the AC is generated around the coil.
The ATC antenna moves above the rails together with the car. As such, when the ATC antenna passes over the ATC ground device, the AC magnetic flux generated at the ATC ground device induces AC voltage in the coils of the ATC antenna. A speed specified by a signal received from the ATC ground device is detected based on voltage that has a specific frequency component in the induced AC voltage and that exceeds a threshold. Then, the detected speed is used as a command speed for performing speed control of a train.
As previously described, the coil of the ATC ground device is loop shaped. As such, the directions of current through the left and right coil sides are opposite to each other and the directions of voltage induced in the left and right ATC antenna coils are also opposite to each other. Therefore, provided that the left and right antenna coils of the ATC antenna are connected in series and are of opposite phase (to have opposite polarities of the voltage induced by magnetic lines oriented in the same direction), the induction voltages each generated at antenna coils are added together to be an amplified voltage, thereby improving speed detection sensitivity.
Current used for propelling other cars flows through the rails on which rolling stock travels. This current is called return current. Magnetic flux occurs around the rails due to this return current. The return current travels in the same direction in the left and right rails. As such, when the left and right antenna coils of the ATC antenna detect magnetic flux caused by return current, the direction of induction voltage is the same for the left and right coils. As previously described, since the left and right ATC antenna coils are connected in series and are of opposite phase, the voltages induced in the left and right coils of the ATC antenna due to the magnetic flux caused by the return current cancel each other out. Thus, the voltage of the specified frequency components is substantially lower than the threshold. This enables erroneous detection of speed signals to be prevented based on the voltage induced by magnetic flux that occurs due to the return current.
Such series connection of the oppositely phased left and right coils of the ATC antenna allows sensitive reception of the ATC speed signals that would otherwise be received under the influence from the return current flowing through the rails.